Nonwoven webs comprising open, lefty, three dimensional structures of fibers bonded to one another at their mutual contact points are used extensively in the manufacture of abrasive articles for cleaning, abrading, finishing and polishing applications on any of a variety of surfaces. Exemplary of such nonwoven articles are those described in U.S. Pat. No. 2,958,593 to Hoover et al. Such nonwoven webs comprise a suitable fiber such as nylon, polyester, blends thereof and the like and are capable of withstanding temperatures at which impregnating resins and adhesive binders are typically cured. The fibers of the web are often tensilized and crimped but may also be continuous filaments formed by an extrusion process such as that described in U.S. Pat. No. 4,227,350 to Fitzer, for example. Nonwoven webs are readily formed on conventional equipment such as a "Rando Webber" machine (commercially available from Rando Machine Company, New York), for example.
Fine abrasive particles (defined herein as particles having a distribution of sizes wherein the median particle diameter in the distribution is about 60 microns or less) may be bonded to the fibers of a nonwoven web to provide abrasive articles suitable for use in any of a variety of abrasive applications, and such articles may be provided in the form of endless belts, discs, hand pads, densified or compressed wheels, floor polishing pads and the like. A particularly appropriate use for articles comprising the aforementioned fine particles is in the automotive aftermarket industry, where the abrasive particles are employed to "scuff" or lightly abrade automobile body panels in preparation for painting. In these applications, the abrasive article is applied to a previously painted surface. During the application, the abrasive particles in the article scratch the surface to reduce the surface gloss to a "haze". Although the commercial success of available abrasive articles has been impressive, it is desirable to further improve the performance of certain abrasive articles especially in applications in the automotive aftermarket, for example.
In the manufacture of these articles, a nonwoven web is prepared, as mentioned. The web is reinforced, for example, by the application of a prebond resin to bond the fibers at their mutual contact points. Additional resin layers may subsequently be applied to the prebonded web. A make coat precursor is applied over the fibers of the prebonded web and the make coat precursor is at least partially cured. A size coat precursor may be applied over the make coat precursor and both the make coat precursor and the size coat precursor are sufficiently hardened in a known manner (e.g., by heat curing). Fine abrasive particles, when included in the construction of the article, are conventionally applied to the fibers in a slurry with the make coat precursor.
Prior to or during the curing of the make coat, the resinous slurry of make coat precursor and fine abrasive particles is known to migrate and to concentrate or agglomerate at the intersection of two or more fibers in the web, or at points where a single fiber crosses itself due to known surface tension effects, for example. The resulting abrasive articles have a substantially nonuniform distribution of the agglomerated resin and the fine abrasive particles along the lengths of the fibers. Further, because the particles are applied to the web in a resinous slurry, the fine abrasive particles tend to become engulfed in the cured resin, as is illustrated in FIG. 1 wherein the resinous adhesive forms agglomerates 12 along the lengths of the fibers 10 of the nonwoven web with the fine abrasive particles dispersed and engulfed within the resin. In such a construction, the fine abrasive particles may not be immediately available in abrading applications of the finished article, possibly making the overall abrasive performance of the articles less than optimum and leaving room for improvement in performance. In the automotive aftermarket industry, for example, the initial unavailability of the abrasive particles can result in an undesirably low initial abrasive action when the article is applied to the surface, prompting the user to exert high pressures on the article during the abrasive operation which may have an undesired effect on the surface being treated.
Historically, lofty, open, 3-dimensional nonwoven abrasive articles have been made using a variety of coating techniques. In the aforementioned U.S. Pat. No. 2,958,593 (Hoover et al.) for example, nonwoven articles were made by the spray application of a relatively dilute slurry comprising a solution of binder, organic solvent and abrasive particles. It was expected that other coating methods and procedures might provide advantages under specific circumstances.
From Hoover et al.:
It should be noted, however, that by employing techniques other than spraying, somewhat greater thicknesses of web may be suitably treated in forming our structures. In fact, roll coating, dip coating, separate application of adhesive and mineral, etc., may have advantages over the spray application described in the previous examples. For instance, spraying the adhesive first and then sifting in the abrasive separately is particularly suitable for incorporating coarse mineral, (e.g. grit 50 or larger), and also results in products of slightly differing abrading characteristics.
With the passage of time, it became desirable to minimize resin waste from overspray and minimize or eliminate volatile organic compounds from use in the manufacturing process. Consequently, the spray coating techniques exemplified by Hoover et al. generally fell into disfavor, and the present day use of roll coating techniques to apply water-based resin/abrasive slurries began in earnest. As the performance characteristics of nonwoven abrasive articles became more demanding, the resin/abrasive coatings employed in the manufacture of nonwoven abrasive articles and methods for the application of such coatings have continued to evolve. However, the foregoing problem of uniformly coating fine abrasive particles onto the fibers of a nonwoven web has persisted.
Efforts to overcome the problem of resin and particle agglomeration in the application of fine abrasive particles to nonwovens include attempted drop coating or spray coating techniques, as taught or suggested by Hoover et al. In these efforts, dry abrasive particles are deposited onto the fibers of the web after the application of the uncured make coat precursor. However, in the deposition of fine abrasive particles by these techniques, the distribution of the particles is greatly influenced by electrostatic forces and ambient moisture conditions which occur naturally in the materials (e.g., the particles) and in the equipment used in the deposition process. As a result of these forces, fine abrasive particles have shown a consistent tendency to agglomerate while still resident within the coating equipment as well as after the particles have been released therefrom. This particle to particle interaction or agglomeration may result in abrasive articles comprising significant particle agglomerates with non-uniform particle distributions within the resulting webs. Such articles may possess nonuniform performance characteristics, and the nonuniformity of the particle distribution, with the presence of particle agglomerates, can create a commercially unacceptable appearance in the article. Moreover, standard roll coating techniques used in the application of the make coat precursor can add excessive amounts of the resin to the web, resulting in resin layers which can readily engulf fine abrasive particles once they are applied to the web.
Accordingly, it is desirable to solve the above described problem and to thereby fulfill a long felt need relating to the optimization of fine abrasive particle distribution in nonwoven abrasive articles. It is desirable to provide nonwoven abrasive articles comprising a nonwoven web with fine abrasive particles adhered to the fibers of the web wherein the particles are distributed along the lengths of the fibers of the web in a substantially uniform manner and wherein an increased percentage of the abrasive particles are immediately available for abrasive applications of the finished article.